This invention relates to an optical modulator and in particular, although not exclusively, to a waveguide type optical modulator.
Optical modulators are extensively used in optical telecommunication systems to modulate light from a laser source. The optical modulator in response to a modulating signal, which is applied to a control input of the modulator, applies a variable attenuation to the intensity of an optical signal passing through it or changes the phase of the optical signal.
Optical modulators can generally be categorised into two types depending on their principle of operation, these being xe2x80x9cresonantxe2x80x9d or xe2x80x9cnon-resonantxe2x80x9d modulators. Both types of modulators can be implemented as waveguide or non-waveguide devices. In a waveguide implementation the optical signal(s) is guided along a specific path, or paths, in a specific waveguide mode(s). Modulation of the optical signal is effected by changing the propagation properties of the mode(s) within the waveguide(s). Such modulators tend to be planar devices such as for example the Mach-Zehnder intensity modulator.
In non-waveguide implementations there is a degree of flexibility in the optical pathway through the device, since there is less spatial control over the optical signal. Modulation is again effected by changing the propagation properties of the optical signal in the propagating medium. Examples of such modulators are Fabry-Perot modulators and liquid crystal spatial light modulators.
Resonant modulators operate by changing the resonant wavelength to effect switching between the resonant and non-resonant state at a particular wavelength. This change is achieved by altering the optical phase change of the signal as it passes through an xe2x80x9cactivexe2x80x9d medium (the active medium being the part of the device over which the modulation effect is applied). Modulation can be achieved by using a wide range of physical phenomena such as electro-optical, electromechanical or thermo-optic effects.
One example of a resonant modulator is the Fabry-Perot vertical cavity reflective modulator which is a non-waveguide device. The device comprises two partially reflecting mirrors separated by an xe2x80x9cactivexe2x80x9d medium which is made of an electro-optic material. At certain resonant wavelengths the optical signal is reflected by the first mirror and the device does not transmit light of this wavelength. At other wavelengths the optical signal passes substantially unattenuated through the first mirror, the xe2x80x9cactivexe2x80x9d medium and the second partially reflecting mirror. Modulation of the device is achieved by applying an electric field to the xe2x80x9cactivexe2x80x9d medium between the mirrors to change the effective refractive index of the xe2x80x9cactivexe2x80x9d medium. This change in refractive index has the effect of shifting the resonant wavelength. Thus for an optical signal of fixed wavelength the modulator can be made to switch between reflective and tranmissive states.
Non-resonant modulators operate by modulating the phase and/or the intensity of the optical signal in the xe2x80x9cactivexe2x80x9d medium within the modulator. This switching can be achieved by a wide range of physical phenomena e.g. electro-optic, electro-absorption, electro-mechanical or thermal effects. An example of such a device is the Mach-Zenhder modulator which is an interferometric waveguide device. In this type of modulator the optical signal is split to pass along two optical paths each of which comprises an xe2x80x9cactivexe2x80x9d medium made from an electro-optic material whose refractive index is dependent upon an applied electrical field. The optical output of the device is derived by combining the outputs from the two optical paths. Modulation of the optical output intensity is achieved by changing the phase difference between the optical signals in the two paths of the device, and thereby changing the intensity of the combined optical output.
Important parameters for all types of optical modulators are (i) its efficiency (that is its drive power requirements, (ii) its physical size, (iii) the optical insertion loss it presents to a system in which it is to be used, (iv) its ease of integration into an optical system, (v) the capability to modulate at high data rates, (vi) its ease of manufacture and (vii) its cost. The design of optical modulators is often a compromise to optimise one or more of these parameters.
The present invention has arisen in an endeavour to provide an optical modulator which is an improvement, at least in part, on the known modulators.
According to the present invention an optical modulator comprises an optical input; an optical output; at least one optical path connecting the optical input and output; means for selectively changing the optical characteristic of a part of the at least one optical path in response to a control signal such as to modulate light passing along the optical path characterised by the optical path including a structure which slows the passage of light along the part of the optical path. Due to the structure within the optical path the interaction time of an optical signal passing along the part of the optical where it is modulated is increased and the modulation effect is thus enhanced.
Advantageously the structure comprises a sequence of coupled resonator structures such as to form an optical slow-wave structure. Conveniently each resonator structure is defined by a pair of partially reflecting planes which define an optical resonator cavity within the part of the optical path. In a preferred implementation each of the partially reflecting planes is defined by a grating structure.
Preferably the optical path comprises an electro-optic material whose refractive index is dependent upon an applied electrical field and wherein the means for changing the optical characteristic comprises electrodes for applying an electric field to the material.
Alternatively the optical path comprises an electro-absorption material whose optical absorption is dependent upon an electrical field and wherein the means for changing the optical characteristic applies an electric field to the material.
As a further alternative, the optical path comprises a thermo-optic material whose optical properties are dependent upon temperature and wherein the means for changing the optical characteristics changes the temperature of the material.
In a preferred implementation the modulator further comprises a second optical path; an optical splitter for splitting the optical signal to pass along the first and second optical paths; an optical combiner for combining the optical signals from the paths to form the optical output wherein each of the optical paths comprises an electro-optic material whose refractive index is dependent on electrical field and the means for changing the optical characteristics applies different electrical fields to the two paths.
Advantageously when the modulator is an electro-absorption or electro-optic device the means for changing the optical characteristics comprises a plurality of electrodes disposed along the, or each, optical path in a direction of propagation of light along the, or each, optical paths such that the electrical field travels along said path or paths and wherein the velocity of the travelling electrical field and slow-wave optical signal are matched.
In any of the embodiments, the optical path(s) is (are) defined by a waveguide.